The present invention relates generally to the determination of the short axis of a heart using an imager and, more particularly, to determining a three-dimensional (3D) coordinate system of a heart using a magnetic resonance (MR) imager.
During MR imaging, when a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Imaging devices, such as MR devices that implement the principles discussed above, have been used to determine a coordinate system of a heart. That is, imaging devices have been employed to determine the short, vertical long, and the horizontal long axis of the heart. Methods for determining axes of the heart each come with their own drawbacks. For example, a variety of methods for determining the axes of a heart often require an imaging subject to withstand breath-hold techniques so that such determinations are accurate. However, many subjects have difficulties withstanding breath-holds for an appropriate length of time. In addition to breath-hold drawbacks, other drawbacks arise from such methods. For example, a substantial amount of operator instruction is often needed to adequately determine one or more heart axes. In such cases, it is often helpful that such an operator is familiar with cardiac anatomy. As such, the accuracy of axis determinations of a heart is often dependent upon the knowledge of an operator.
It would therefore be desirable to have a system and apparatus that overcomes such aforementioned drawbacks.